In a mobile communication system such as an HSDPA (High Speed Downlink Packet Access) system, a user equipment terminal (UE) measures reception quality of a pilot channel and periodically reports channel quality information (CQI: Channel Quality Indicator) to a base station (at transmission intervals specified by the base station). The base station determines a modulation scheme, a coding rate, or the like based on the reported CQI (see 3GPP, TS25.214).
FIG. 1 shows CQI information transmitted in accordance with HSDPA. For example, the user equipment terminal measures reception quality of the pilot channel and reports a numerical value of 0-31 representing the reception quality to the base station. Specifically, the CQI (CQI_a1, CQI_a2, CQI_a3) is a value (absolute value) representing the channel quality with a predetermined number of bits (for example, five bits). When the base station receives a false CQI, the base station determines a false modulation scheme or a false coding rate. However, when the base station subsequently receives a correct CQI, the base station can determine a correct modulation scheme or a correct coding rate.